rpcrt4: Fix the retrieval of This for interpreted proxies.

dlls/rpcrt4/ndr_stubless.c

@@ -578,7 +578,7 @@ LONG_PTR WINAPIV NdrClientCall2(PMIDL_STUB_DESC pStubDesc, PFORMAT_STRING pForma
     if (pProcHeader->Oi_flags & RPC_FC_PROC_OIF_OBJECT)
     {
         /* object is always the first argument */
-        This = *(void **)ARG_FROM_OFFSET(stubMsg, 0);
+        This = **(void ***)(&pFormat+1);
         NdrProxyInitialize(This, &rpcMsg, &stubMsg, pStubDesc, procedure_number);
     }
     else